We have observed previously that newly synthesized DNA in UV-irradiated mitotic and mieotic yeast cells is substantially larger than the distance between pyrimidine dimers indicating that synthesis can take place past dimers. We have been investigating the ability of pyrimidine dimers to block or allow DNA replication in yeast using the method of Moore and Strauss (Nature 278 (1969) 664) which utilizes in vitro synthesis on primed 0X174 DNA templates that have been UV-irradiated. We have modified this procedure in order to sequence the DNA products synthesized with various polymerases, yeast crude cell lysates or combinations of these. Synthesis of DNA by the crude extract system or purified yeast DNA polymerase I is terminated by dimers and the extracts do not appear to increase the possibility of read-through by exogenously added purified DNA polymerases (E. coli polymerase I and yeast DNA polymerase I). Using these procedures we are quantitating the extent of termination by DNA damage and comparing the relative synthesis in extracts from mitotic and meiotic cells or cells stressed with UV-irradiation.